This invention relates to a method of controlling a traffic control system comprising, arranged along a traffic lane, at least two mutually spaced measuring points and a signalling arrangement located between the measuring points, the method comprising the steps of detecting the speeds V.sub.c of vehicles passing the measuring points, determining a running weighted average speed V.sub.g from detected vehicle speeds V.sub.c, determining whether a detected vehicle speed is less than a predetermined part of the running weighted average speed V.sub.g and applying an alarm signal to the signalling arrangement when the detected vehicle speed is less than the predetermined part of the running weighted average speed V.sub.g.
Such a method is described in the article "Tunnel and motorway supervision system REYERS-Brussels", by D. W. Singleton and H. H. A. Heesterbeek, published in Philips Telecommunication Review, Vol. 32, December 1979, pages 246-257.
The method described in said article is performed with the aid of a central processor. In this processor the vehicle speed V.sub.c detected at a measuring point is compared with a predetermined percentage of a running weighted average speed, composed of vehicle speeds originating from a plurality of consecutive measuring points arranged along a traffic lane.
It has, however, been found that such a method is inaccurate because, on the basis of the above-mentioned criterion, an alarm signal is generated too often. This results in an excessive number of alarm signals to be applied to the signalling arrangement which is a source of irritation for drivers and consequently reduces the traffic safety. In addition, the traffic speed is reduced unnecessarily, which has an adverse effect on the traffic flow.